Good evening, @ROBINKRAUSE ...
I'll just add to @greydon's response that while it technically would be possible to be enrolled as both a Teacher and a Student in the same course (I think you just add the user again with a different role), it is not something that I would recommend doing...especially if your course uses third-party LTI integrations. I recall testing this a few years back in a course for one of our instructors (as he wanted to see what things looked like from a student perspective). However, when the instructor was enrolled in the course as both a Teacher and a Student, his access to the third-party LTI tool (I think in this case, it was Turnitin or some other book publisher tool) became more limited. I think the LTI tool got confused as to what role the instructor really had in the course, and so it was having difficulties displaying the LTI content. As soon as I removed the Student role from his course in Canvas, he was able to access the LTI tool again as an instructor without issue.
I hope this extra bit of information will be of help to you in some way.